348 PHILOSOPHICAL TRANSACTIONS. [aNNO 166q» 



It is manifest, that by this way you find exactly enough the longitude of the 

 place where you were at noon, or the time of the sun's being in the south ; 

 which, though it may differ from the longitude of the place where you are 

 when you observe the setting of the sun, yet you may estimate near enough how 

 much you have advanced or changed the longitude in those few hours, by the 

 log-line, or other ordinary practices of reckoning the ship's way; or, which is 

 the surer way, by the degrees passed in 24 hours by a former day's obser- 

 vation. 



You may also, instead of observing the sun's rising and setting, observe the 

 setting first, and then next morning the rising; marking at both times the time 

 shown by the watches ; and find thence, after the same manner as before, the 

 longitude of the place where the ship was at midnight. 



~ Finally, you may also, instead of the rising and setting of the sun, observe 

 before and after noon two equal altitudes of the sun, noting the time shown by 

 the watches, and reckoning in the same manner as has been said of the rising 

 and setting. Yet it is to be considered, that the sun's altitudes are best taken 

 when he is about east and west, as has been already intimated. But note, that 

 in sailing north or south, you take not the observations at the sun's rising and 

 setting, but when he is due east and west. 



9. But you may put the rule in practice, by taking two equal altitudes of 

 some known star, that rises high above the horizon. For you will thence, 

 according to the rule, know at what time, by the watches, the star has been in 

 the south, and so the star's right ascension being known, as also that of the 

 sun, you may thence easily calculate what time it then was, which being com- 

 pared with the time of the watches, as before, will give the longitude of the 

 place where you were when you had the star in the meridian. 



10. If the watches that have gone exactly for a while, should afterw^ards differ 

 from one another, as in length of time it may well happen ; in that case it will 

 be best to reckon by that which goes fastest; unless you perceive an apparent 

 cause why it goes too fast ; seeing it is not so easy for these pendulum watches 

 to move faster than at first as it is to go slower. For the wire on which the 

 pendulum hangs may perhaps, by the violent agitation of the ship, come to 

 stretch a little, but it cannot grow shorter, and the little weight of the pendu- 

 lum may perhaps slip downwards, but cannot get up higher. 



1 1 . When you get sight of any known country, island, or coast, be sure to 

 note their longitude, as exactly as you can by help of the rules here prescribed ; 

 that thereby you may correct the sea maps ; and that you may know how far you 

 have sailed from any place, to the east or west. And if by any accident the 



